Scanning electron microscope and method for detecting an image using the same

ABSTRACT

In the present invention, in order to realize both a reduction of an image detecting time and high quality image detection in a scanning electron microscope for measurement, inspection, defect review, or the like of semiconductor wafers, a low-magnification image is taken by using a large beam current; a high-magnification image is taken by using a small beam current; control amounts for correcting a change in luminance, a focus deviation, misalignment, and visual field misalignment of taken images, which are generated due to a variation of a beam current are saved in advance in a memory of an overall control system; and these amounts are corrected every time the beam current is switched, thereby making it possible to take the images without any adjustment operation after switching the currents.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese patent applicationNo. JP 2005-005882 filed on Jan. 13, 2005, the content of which ishereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning electron microscope(hereinafter, referred to as “SEM: Scanning Electron Microscope”) and animage detecting method thereof for detecting an image of a surface of anobservation target by irradiating a convergent electron beam to theobservation target such as a semiconductor wafer and detecting electronsemitted from the irradiated position, and particularly relates to atechnique effectively applied to an SEM semiconductor wafer inspectionapparatus required to take high-magnification images, a review SEM forobserving in greater detail defects detected in the semiconductor wafer,and a length measurement SEM for measuring a pattern formed on thesemiconductor wafer, etc.

As a technique that the present inventors have studied, the followingtechniques are conceivable in, for example, the SEM semiconductor waferinspection apparatus, and the review SEM, the length measurement SEM,etc. (for example, see Japanese Patent Laid-Open Publication No.2003-16983).

Along with miniaturization of the patterns on the semiconductor wafer,control of a front-end manufacture process of semiconductor is becomingmore and more difficult; detection of defect, observation, and dimensionmeasurement of pattern width in the optical microscope are becomingdifficult; and inspection, review (reexamination), and lengthmeasurement are performed based on the images taken by the SEM.

The SEM emits an electron beam converged on a surface of the observationtarget, and detects a secondary electron or reflected electron emittedfrom the irradiated position. When the irradiated position of theelectron beam is two-dimensionally moved, a two-dimensional image istaken.

Expected values of the secondary electron and the reflected electronsthen emitted upon irradiation of the electron beam are known to beproportional to a beam current which controls an amount of electronbeams to be irradiated. The number of the emitted secondary electrons isnot completely the same in the case of the same beam current, avariation occurs in the emitted number, and the variation isproportional to the emitted electron number to one-half power. Thisvariation is generally known to be a main factor of noise of thedetected image in SEM.

Regarding an S/N of detected images, the symbol “S” indicating a signalis proportional to the beam current and the variation which is a noisecomponent is proportional to the beam current to one-half power, so thatthe S/N is proportional to the beam current to the one-half power. It isknown that, for this characteristic, in order to obtain the image withthe good S/N, the image has to be detected by use of the beam currentwhich is as large as possible.

However, it is known that, when the beam current is increased,aberration of an electro-optical system is increased and a beam diameteris increased. Therefore, there has been a problem that high-resolutionimages cannot be obtained. As a technique for solving this problem, anoise reduction technique which is performed by addition through a frameis known. This is a method in which a plurality of images within thesame region are taken and each of the images is used as a frame and theframes are integrated to synthesize a final image. When this method isemployed, the S/N is proportional to the number of added frames to theone-half power, while the image taken time increases in proportion tothe number of added frames.

By the SEM having such a characteristic, defects of the semiconductorare reviewed or the dimensions of the semiconductor pattern aremeasured. However, when such processes are to be performed, two types ofimages are generally taken, i.e., low-magnification image detection andhigh-magnification image detection are performed. For example, in thereview SEM for reviewing the defects of semiconductor, the defect isenlarged at high magnification and displayed in accordance with defectcoordinates outputted by an inspection device for detecting the defects.However, accuracy of the defect coordinates outputted by the inspectiondevice is bad within a visual field of the high-magnification image.Therefore, first, a defect position is specified by comparing thelow-magnification defect image with a reference image which is a normalimage having the same pattern as that of the defect image, and thedefect position is enlarged to obtain the high-magnification image.

On the other hand, in the length measurement SEM, in order to determinea pattern to be measured, a low-magnification image is similarlydetected. The image is taken subsequently at high magnification in orderto perform measurement with high accuracy, and line width etc. ismeasured from the taken image.

SUMMARY OF THE INVENTION

Incidentally, the present inventors have studied the technique of theSEM as described above and, as a result, the following has beenapparent.

That is, in the above-described conventional techniques, the beamcurrent at the low-magnification image detection and the beam current atthe high-magnification image detection have to be the same. Therefore,there has been a problem that when a magnification difference betweenthe high-magnification image and the low-magnification image is large,an image taken time of the low-magnification image becomes longer.

Hereinafter, an example of the review SEM will be described. Althoughthe low-magnification image is used as an image employed for detectingthe defects. However, since defect-detection coordinate accuracy of theinspection device with respect to size of the defect is bad, thehigh-resolution image has to be taken in a large visual field. Even if apitch of semiconductor pattern is reduced to 65 nm in the future, thesize of defect which has to be detected within the visual field thereofis conceived to be about 25 nm, which is sufficiently large whencompared with the resolution of a general SEM, for example, with 4 nm.

Meanwhile, the defect-detection coordinate accuracy in the inspectiondevice is particularly bad in a dark-field inspection device, forexample about ±4 μm in accuracy. For this reason, a defect with about 25nm is required to be detected from an 8-μm square visual field. However,when the image is taken by sampling the visual field by 512×512, thedefect is detected in about 1.5 pixels, so that a high defect capturingrate cannot be expected. In order to improve the defect capturing rate,a method of, for example, doubling a density of sampling is required.However, in this case, taking the image by the same S/N requires a timethat is four times longer.

On the other hand, when the sampling is performed with a doubleddensity, i.e., at 1024×1024, the size per pixel becomes about 8 nm whichis twice or more of the resolution of the SEM having been describedabove as an example. Therefore, even when a beam spot diameter issomewhat increased by increasing the beam current, the image can betaken without deteriorating the resolution in the taken image.

However, there is a problem that, when the same beam current is used fortaking the high-magnification image, the image of higher resolutioncannot be obtained. For example, when the defect of about 25 nm is to betaken in a size of about one eighth of the image, the image detection ina 200-nm square visual field is required. When this is sampled by512×512, the size per pixel becomes about 0.4 nm. This is a resolutionthat the general SEM cannot reach and, moreover, in a state in which thebeam current is large, the resolving power is apparently insufficient.

Thereat, although a method of setting an optimum beam current for eachof the low-magnification image and the high-magnification image isconceivable, the conventional method has been incapable of setting theabove optimum beam current.

A reason for that includes a first problem of an occurrence ofmisalignment of visual fields after being shifted between the small beamcurrent and the large beam current and misalignment of the beam. In thehigh-resolution SEM, generally, a FE (field-emission) electron gun or aschottky emission electron gun is employed as an electron source. Inorder to change the beam current by the FE electron gun, the extractionvoltage of the FE electron gun is changed. When the extraction voltageis changed herein, an assumed light source position is changed, wherebythe visual field misalignment and misalignment in the axis of theelectron gun occurs. Therefore, in order to take the good image in anaxis-adjusted state, adjusting the alignment of the axis has beenrequired every time the beam current is changed.

A second problem is a switching time of the current. When shortening animage taken time of the low-magnification image is considered to be amain purpose of beam current switching, the switching of the current hasto be carried out at an at least shorter time than the image taken timeof the low-magnification image. Generally, the image taken time of theSEM is about 640 ms in terms of 16 frame additions in an image samplingof 512×512. In order to ensure the same S/N in a sampling of 1024×1024,four times the image taken time of the SEM, i.e., 2560 ms is required.Herein, when two images, i.e., a defect image and a reference imagewhich it is known that previously has the same pattern as that of thedefect image are taken at the low magnification, it takes 5120 ms. Ashortened time in the case where the beam current is increased fourtimes and the low-magnification defect image and reference image aretaken 1280 ms becomes 3840 ms, so that the beam current has to beswitched within an at least shorter time than 3840 ms.

However, when the extraction voltage of the FE electron gun is changed,a period of time is generally consumed more than until the beam currentis stabilized and the overall image taken time has not been shortenedeven when the beam current is switched.

Moreover, the beam current can be changed by controlling a suppressorvoltage in the case of using the schottky emission electron gun. Also inthis case, however, similarly to the case where the extraction voltageof the FE electron gun is changed, there is a problem that a period oftime is consumed until it is stabilized.

An object of the present invention is to realize both shortening of theimage taken time and obtaining of the high-quality image detection inthe SEM.

The above and other objects and novel feature of the present inventionwill become apparent from the description of the present specificationand the accompanying drawings.

Outlines of representative ones of inventions disclosed in the presentapplication will be briefly described as follows.

More specifically, an SEM according to the present invention has: acontrol amount memory means for saving, in advance, an adjustment amountof a gain adjusting means for making a detection value of an electrondetector falling within a predetermined range in setting a plurality ofbeam currents, or a calculation algorithm of the adjustment amount; andan image detecting condition control means for changing an imagedetecting condition in accordance with a change of the beam current byusing the adjustment amount or calculation algorithm of the adjustmentamount saved in the control amount memory means.

In addition, an image detecting method of an SEM according to thepresent invention includes a first image detecting step of making anelectron beam converged into a spot-like shape being scanned on anobservation target surface, and converting a secondary electron orreflected electron generated in the scan into an electrical signal so asto form an image; a defect position specifying step of specifying adefect position on the observation target surface based on the imageformed in the first image detecting step; a second image detecting stepof making an electron beam converged into a spot-like shape beingscanned at a defect position specified in the defect position specifyingstep by using a magnification larger than the magnification in the firstimage detecting step, and converting a secondary electron or reflectedelectron generated in the scan into an electrical signal so as to forman image; a first beam current changing step of observing theobservation target surface in the second image detecting step by using abeam current smaller than a beam current of the first image detectingstep; a first image detecting condition setting step of changing animage detecting condition to a condition set in advance for detecting animage by using the beam current set in the first beam current changingstep; a second beam current changing step of setting the beam currentemployed in the first image detecting step; and a second image detectingcondition setting step of setting the image detecting condition employedin the first image detecting step again, wherein the first beam currentchanging step and the first image detecting condition setting step areperformed before the second image detecting step and the second beamcurrent changing step and the second image detecting condition settingstep are performed after the second image detecting step.

According to the SEM and the image detecting method according to thepresent invention, the electron beam can be switched within a shortperiod of time, and both the short-time low-magnification imagedetection and the high-resolution high-magnification image detection canbe performed.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a configuration of an SEM according to anembodiment of the present invention.

FIG. 2 is a flow chart showing a sequence of the case where defectimages are automatically taken in the SEM according to the firstembodiment of the present invention.

FIG. 3 is a flow chart showing a sequence of the case where lengthmeasurement of patterns is performed in the SEM according to the firstembodiment of the present invention.

FIG. 4 is a view showing a configuration of an SEM according to anothermethod of the present invention.

FIG. 5 is a view showing a configuration of an SEM according to anotherembodiment of the present invention.

FIG. 6A is a view showing an energy distribution of spatial frequency inwhich pixel size of an entire low-magnification image is used as areference.

FIG. 6B is a view showing an energy distribution of spatial frequency inwhich pixel size in an entire high-magnification image is used as areference.

FIG. 7 is a view showing an example of a structure of an aperture stopset in the SEM shown in FIG. 4.

FIG. 8 is a view showing a configuration of an SEM according to anotherembodiment of the present invention.

FIG. 9 is a view showing an example of a GUI for inputting image takenconditions in a sequence of a defect image detection shown in FIG. 2.

FIG. 10 is a view showing an example of a GUI for inputting image takenconditions in a sequence of pattern length measurement shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be detailed basedon the drawings. Note that, throughout all the drawings for explainingthe embodiments, the same members are denoted in principle by the samereference numeral and the repetitive description thereof will beomitted.

In a semiconductor wafer, patterns are formed in a multi-layeredstructure through many steps. In order to monitor a manufacture processin a flow of producing the multi-layered structure, a dimensionalmeasurement of a pattern formed per layer, a visual inspection of thepatter, and a review (reexamination) of a defect detected in the visualinspection have been performed.

A recent semiconductor process is increasingly made fine. Therefore, aSEM capable of take an image with resolution higher than that of anoptical microscope has been applied as an image detection for performingthe process. A review SEM has been widely used as the SEM employed forsuch a purpose. A main function of the review SEM is to move a visualfield to a defect position in accordance with the coordinates of thedefect which has been detected in the visual inspection and to take animage of the defect by the SEM.

FIG. 1 is a view showing a configuration of an SEM according to anembodiment of the present invention. First, an example of theconfiguration of the SEM according to the present embodiment will bedescribed with reference to FIG. 1.

The SEM of the present embodiment is, for example, a review SEM and iscomprised of an electron beam source 101, condenser lenses 102 and 103,an electron beam axis adjuster 104, scanning units 105 and 106, anobjective lens 107, an E×B (E cross B) 110, an electron detector 111, anA/D converter 112, a memory 113, an XY stage 114, an image processingunit 115, a secondary storage device 116, a computer terminal device117, and an overall control system 118, etc.

An operation of the SEM according to the present embodiment will next bedescribed with reference to FIG. 1.

The electron beam source 101 emits an electron beam. The emittedelectron beam passes through the condenser lenses 102 and 103, and thenastigmatism and misalignment are corrected by the electron beam axisadjuster 104. The electron beam is deflected by the scanning units 105and 106 so as to control a position where the electron beam isirradiated. Then, the electron beam is converged by the objective lens107 to irradiate an image detecting target 109 of a wafer 108. As aresult, a secondary electron and a reflected electron are emitted fromthe image detecting target 109, and the secondary electron and reflectedelectron are deflected by the E×B 110 and detected by the electrondetector 111. The secondary electron and reflected electron detected bythe electron detector 111 are converted into digital signals by the A/Dconverter 112 and stored in the memory 113. An XY stage 114 moves thewafer 108, thereby making it possible to take images at some positionsof the wafer 108. The image processing unit 115 detects a defectposition based on the image stored in the memory 113. A detection methodthereof includes a method in which the image at the defect position iscompared with an image at a reference position, which it is expectedthat has the same pattern as that of the image of the defect position,and which detects as a defect a position where a difference between bothimages exists. The secondary storage device 116 can store the imagestored in the memory 113. The computer terminal device 117 can displaythe image stored in the secondary storage device 116 or the memory 113.Also, the user can carry out setting of various operations of theterminal device by performing an input to the computer terminal device117. The overall control system 118 controls axis adjustment of theelectron beam, deflection of the electron beam performed by the scanningunits 105 and 106, and visual field transfer by movement of the XY stage114.

A sequence of taking an image defect by the SEM according to the presentembodiment will next be described with reference to FIG. 2. FIG. 2 is aflow chart showing a sequence of taking the defect image in the SEMshown in FIG. 1. The following operations are basically automaticallyperformed in accordance with control executed by, for example, theoverall control system 118, wherein its portion can be manually executedvia, for example, the computer terminal device 117.

First, a beam current for taking a low-magnification image is set instep S201. Then, in step S202, alignment of the axis is adjusted for thecurrent, astigmatism is corrected, and a control value for performingthis correction is saved in a memory in the overall control system 118.The correction may be performed manually, or a method disclosed inJapanese Patent Laid-Open Publication No. 2003-16983 may be employed.

Then, an image is taken in step S203. Next, a beam current for taking ahigh-magnification image is set in step S204, the axis at the beamcurrent for taking the high-magnification image is aligned in step S205,astigmatism is corrected, and a control amount for performing thiscorrection is saved in the memory in the overall control system 118.

In step S206, an image is taken by the beam current for taking ahigh-magnification image. In step S207, the position of the image takenin step S203 and that of the image taken in step S206 are aligned, and aposition misalignment amount is saved in the memory in the overallcontrol system 118.

In step S208, a defect to take an image is selected. In step S209, theXY stage 114 is moved, whereby a reference position corresponding to theselected defect enters the visual field of the SEM. At the same time,the beam current is switched in step S219 (second beam current changingstep), and setting of the electro-optical system for taking thelow-magnification image is performed in step S220 (second imagedetecting condition setting step). More specifically, a normal controlamount of the electro-optical system for taking the low-magnificationimage is set, and concurrently the alignment of the axis of the beamcurrent for taking the low-magnification image and the control amountfor astigmatism correction, which have been saved in step S202, are set.

In step S210, a reference image is taken. In step S211, the XY stage 114is moved so that the defect selected in step S208 enters the visualfield of the SEM.

The image is taken in step S212 (first image detecting step), and aposition of the defect is specified in step S213 (defect positionspecifying step) by comparing the image taken in step S210 and the imagetaken in step S212 in step S213 (defect position specifying step). Atthe same time, the beam current is varied for high-magnification imagein step S214 (first beam current changing step), and setting of theelectro-optical system for taking the high-magnification image isperformed in step S215 (first image detecting condition setting step).More specifically, a normal control amount of the electro-optical systemfor taking the high-magnification image is set, and the alignment of theaxis of the beam current for taking the high-magnification image and thecontrol amount for the astigmatism correction, which have been saved instep S205, are set.

In addition, in step S216, based on misalignment amounts of thelow-magnification image detection and the high-magnification imagedetection measured in step S207, an electron beam irradiating positionwhere the defect is taken at a center of the visual field is calculatedwhen the electron beam is emitted.

In step S217, an AF (automatic focus) is performed. In step S218 (secondimage detection), the high-magnification image is taken. This image isstored in the memory 113 or the secondary storage device 116.

In step S221, it is checked whether all points of the image are taken.If images of all the points have been taken, the process is ended. Ifthe images of all the points have not been taken, the process returns tostep S208 and the steps S208 to S221 are executed again.

By using the above-described method, the low-magnification images andhigh-magnification images can be taken without being affected by themisalignment or astigmatism variation caused when the beam current ischanged.

Also, when the beam current of the electron beam source 101 is changed,the position where the electron beam is focused is changed depending ona configuration of the electro-optical system in some cases. Therefore,handling of such cases may be carried out by: performing the AF in stepS202 and step S205; serving the control amount in which the focuses ofthe respective beam currents coincide with one another; and beingcorrected by the saved control amount so that the focuses in step S220or step S215 coincide with each other.

Generally, the electron detector 111 comprises a combination of ascintillator and a photomultiplier in many cases. When the beam currentis varied for the low-magnification images and high-magnificationimages, the number of detected electrons varies, so that adjustment of again of the photomultiplier is required. Therefore, the gain of thephotomultiplier which enables the respective beam currents to take thegood images may be obtained in step S202 and step S205 and, at the sametime, the gains suitable for the respective beam currents in step S215and step S220 may be set.

It should be noted that saving previously the control amounts suitablefor the respective beam currents has been described, an expression forcalculating values of the control amounts may be saved so as to set thecontrol amounts in step S220 and S215 based on the expression. Forexample, as for the gain of the photomultiplier, since it is understoodin advance that the beam current is approximately proportional to thenumber of secondary electrons and reflected electrons generated by thecurrent, a method of making the gain of the photomultiplier inverselyproportional to the beam current may be used.

The image detecting conditions in step S219 and step S214 have beendescribed in the description of FIG. 2. However, if the beam current fortaking the low-magnification image and the beam current for taking thehigh-magnification image, which are set herein, are configured to be setby the user through the computer terminal device 117, however, usabilityis improved. At this time, for example, a GUI (Graphical User Interface)as shown in FIG. 9 is made to be displayed in the computer terminaldevice 117, whereby the image detecting conditions are set. The item“probe current” in FIG. 9 corresponds to the beam current in the presentspecification. In addition to the probe current, “Field of View”representing the visual field of taking the image, the number of pixelsper image, “image size” indicating, for example, 512×512 or 1024×1024,and “frame integration number” representing the number of times ofscanning the beam with respect to the visual field, etc. can be set foreach of the high magnification and the low magnification. Moreover,since an acceleration voltage cannot be rapidly varied in general, acommon voltage is set herein for the low magnification and the highmagnification.

The sequence of the case where the electro-optical system of FIG. 1 isapplied to an automatic defect image detection of the review SEM hasbeen described in FIG. 2. However, a sequence of the case where thepresent function is applied to a length measurement SEM is shown in FIG.3. FIG. 3 is a flow chart showing the sequence of the case where the SEMshown in FIG. 1 is applied to the length measurement SEM.

In FIG. 3, steps S301 to S307 are the same as the above-described stepsS201 to S207 in FIG. 2 and so their descriptions will be omitted.

Positions to be subjected to length measurement are selected in stepS308, and the XY stage 114 is moved in step S309, thereby causing lengthmeasurement points selected in step S308 to enter the visual field ofthe low-magnification image. At the same time, the beam current isswitched in step S310, and setting of the electro-optical system fordetecting the low-magnification image is performed in step S311. Morespecifically, the normal control amount of the electro-optical systemfor taking the low-magnification image is set, and the alignment of theaxis of the beam current for taking the low-magnification image and thecontrol amount for astigmatism correction, which have been saved in stepS302, are set.

The image is taken in step S312. In step S313, the image taken in stepS311 is compared to an image capable of specifying previously storedmeasurement points, whereby the image detecting point of thehigh-magnification image is specified. At the same time, the beamcurrent is varied for taking the high-magnification image in step S314,and the electro-optical system for taking the high-magnification imageis further set in step S315. More specifically, the normal controlamount of the electro-optical system for taking the high-magnificationimage is set, and the alignment of the axis of the beam current fortaking the high-magnification image and the control amount forastigmatism correction saved in step S305 are set.

Also, in step S316, based on the misalignment amount of thelow-magnification image detection and the high-magnification imagedetection which are measured in step S307, the electron beam irradiationposition where the measurement pattern is imaged at a center of thevisual field when the electron beam is emitted.

The AF is performed in step S317, the high-magnification image is takenin step S318, and a portion in which the pattern is specified ismeasured from the high-magnification image taken in step S319.

FIG. 10 shows a GUI for setting the beam current and the image detectingconditions in the sequence of FIG. 3. In the length measurement SEM,since a portion to be measured is determined in advance, thelow-magnification or high-magnification image can be confirmed so as tomake a confirmation of whether an expected recipe can be confirmed. Amethod for setting image detecting conditions other than this isbasically the same as that of FIG. 9.

Regarding a relation between the beam current and a spot diameter,chromatic aberration which is a main factor for determining the spotdiameter is proportional to the beam current approximately to one-halfpower by an increase in an open angle of the beam involved in anincrease in a beam current value although depending on the beam current.Therefore, in taking the high-magnification image, the beam current issuppressed small to reduce the spot diameter, whereby thehigh-resolution image is taken.

On the other hand, in the low-magnification image detection, when a finedefect is to be detected from a wide visual field or a position to bemeasured is to be accurately determined, the sampling number of imagesis increased from the general sampling number of 512×512 to, forexample, 1024×1024 to perform sampling. In this case, when the beamcurrent is increased by about four times to take the image, a decreasein the S/N due to a decrease in the signal amount per pixel issuppressed. When the beam current is increased by four times, thechromatic aberration is also increased by about two times. However, noproblem is caused in the image quality as long as the accordinglyenlarged spot diameter is smaller than the pixel size. When thelow-magnification image is taken in the above-described manner, theframe integration number can be reduced and the image detecting time canbe shortened.

Note that since the defect position has been already confirmed throughthe low-magnification image detection, taking the image in the widevisual field is not required at a time of the high-magnification imagedetection, so that taking the image thought the sampling of 512×512preferably shortens the image detecting time.

A problem of shortening the image detecting time in the review SEM andthe length measurement SEM is about the beam current switching time.Generally, the switching time takes several tens of seconds in order toswitch the current by changing the extraction voltage of the FE electrongun, that is, becomes longer than the shortening of the image detectingtime of the case where the beam current is increased. Therefore, insteadof changing the extraction voltage of the FE electron gun, a method forchanging the beam current by changing the aperture stop of theirradiation path of the electron beam will be described.

FIG. 4 is a view showing a configuration of an SEM according to theabove method. When the SEM shown in FIG. 4 is compared with theabove-described embodiment shown in FIG. 1, an aperture stop set 403, afield stop 404, and a stop changing means 405 are added and the electronbeam source 101 is replaced by an electron beam source 401 and thecondenser lens 102 is replaced by a condenser lens 402 and otherconfigurations are the same as those of FIG. 1.

A plurality of stops having different diameters are provided in theaperture stop set 403, and the stop to be used can be selected by thestop changing means 405. When the small-diameter stop is used, the beamcurrent becomes small. When the large-diameter stop is used, the beamcurrent becomes large. When the image is to be taken at the lowmagnification, the large-diameter stop is used to increase the beamcurrent. When the image is to be taken at the high magnification, thesmall-diameter stop is used to reduce the beam current. The position ofthe stop is mechanically changed by the stop changing means 405.However, the switching of the beam current within a range of 100 ms canbe realized by virtue of employing the present method.

Note that although the stops are circular apertures with differentdiameters in the embodiment of FIG. 4, stops having shapes other thanthe shapes of apertures can be also employed. Such an example includes amesh (net)-like stop shown in FIG. 7. For example, as shown in FIG. 7,an aperture stop 701 and mesh-like stops 702 and 703 are provided in theaperture stop set 403. The meshes provided in the aperture stop set 403have different electron beam transmittance. When the mesh having hightransmittance (for example, stop 703) is selected, the large beamcurrent is set. When the mesh having low transmittance (for example,stop 702) is selected, the small beam current is obtained. Even when thetransmittance is controlled by combining a plurality of aperturesinstead of the meshes, the same effects can be obtained.

In the embodiment of FIG. 4, an example for changing the beam current bymechanically changing the positions of the stops by the stop changingmeans 405 has been described. However, in this method, dust generateddue to the mechanical movement in the case where the aperture stop set403 is to be moved at a higher speed may be problematic. Thereat, anembodiment in which stops suitable for corresponding to the beamcurrents can be electrically selected is a SEM shown in FIG. 5.

When the SEM shown in FIG. 5 is compared with the above-describedembodiment shown in FIG. 4, the condenser lenses 402 and 103, theaperture stop set 403, the field set 404, and the stop changing means405 are replaced by first condenser lenses 501 and 502, a secondcondenser lens 503, an aperture stop set 504, and deflectors 505, 506,507, and 508, and other configurations are the same as those of FIG. 4.

The electron beam generated by the electron beam source 401 is convergedby the first condenser lenses comprised of a combination of thereference numerals “501” and “502”, and is guided to the objective lens107 through the second condenser lens 503. Stops with a plurality ofapertures are formed in the aperture stop set 504. In the deflector 505,a voltage that causes the electron beam to be deflected so that theelectron beam enters the stop which realizes the selected beam currentis set. Then, a voltage is applied to the deflector 506 in an oppositedirection so that the electron beam is incident in a directionorthogonal to the aperture stop set 504. Also, the deflector 507 and thedeflector 508 causes the axis deflected for selecting the stop to returnto the original axis of the electro-optical system in accordance with amethod similar to that by the combination of the deflector 505 and thedeflector 506. Other configurations are the same as those of theembodiment of FIG. 1, so that the description thereof will be omitted.

In the present method, the beam current can be changed merely bychanging voltages of electrodes of the deflectors 505, 506, 507, and508, so that no dust is generated and switching can be also performed ata high speed.

The embodiment of the case where the aperture stop set having theplurality of beam diameters is employed has been described above.However, providing the plurality of aperture stops is not required tocontrol the amount of the beam passing through the stop. Such anembodiment is shown in FIG. 8.

In a SEM shown in FIG. 8, a stop 801 is disposed instead of the aperturestop set 403 in FIG. 4, and a condenser lens 802 is added. A stop amountof the beam at the position of the stop 801 can be adjusted by changingpower of an electromagnetic lens of the condenser lens 103. When thebeam width at the stop 801 is smaller than an aperture provided in thestop 801, the entire electron beam passes through the beam width. Whenthe beam width is larger than the aperture, vignetting is caused and thebeam current is suppressed to a low level. The position of the beam pathvaried in accordance with the power of the condenser lens 103 can becorrected by causing the condenser lens 802 to vary in combination withthat.

A problem for realizing the SEMs shown in FIGS. 4, 5 and 8 is anoccurrence of contamination (staining) in the stops. When the beamcurrent is set low by the stops, the “vignetting” at the stops isconsiderably large compared with the conventional SEMs. Therefore, a gaspresent in a minute amount in vacuum may be burnt onto and attached tothe stops due to an influence of the electron beam, and the quality ofthe image to be taken may possibly be changed along with time. A meansfor preventing this includes a method of increasing vacuum degree, andfurther includes another method of heating the aperture stop sets 403and 504 so that no contaminant adheres to portions of the stops.

Incidentally, according to the above-described methods, an image havinga high S/N can be obtained in the low-magnification image even if thebeam current is large, but an image with a high S/N cannot be obtainedin the high-magnification image if the beam current is not made small.However, this does not cause any big problem in practice.

For example, as for the automatic image detecting sequence of the defectimages in the review SEM shown in FIG. 2, after the low-magnificationimages are taken in the step S212, the position of the defect has to bedetected through comparison of the images in the step S213. On the otherhand, after the high-magnification image of the defect is taken in thestep S218, this image is stored in the secondary storage device 116 orthe like and then an image of a defect to be the next target is taken.Thereafter, an image processing is performed, whereby the S/N of thehigh-magnification taken image is improved. In the low-magnificationimage, although an image processing for improving the S/N, which takes acalculation time, is difficult to be performed before the step S213since the overall throughput are lowered. Meanwhile, as for thehigh-magnification image taken in the step S218, this can beimplemented. Furthermore, the high-magnification image taken in the stepS218 has the feature of readily performing a processing for improvingthe S/N.

This reason will be described with reference to FIGS. 6A and 6B. FIG. 6Ashows an energy distribution of spatial frequency, in which pixel sizein the entire low-magnification image is used as a reference. Meanwhile,FIG. 6B shows an energy distribution of spatial frequency which is thesame as that in the high-magnification image.

Since the low-magnification image and the high-magnification image takethe images with the same pattern, the energy image in thehigh-magnification image is offset to a band with the low spatialfrequency as compared with the spatial frequency in which each pixelsize is used as a reference. Meanwhile, the reference numerals “603” and“604” represent energy distributions of the spatial frequencies ofnoise. The noise shows a distribution generally called white noise anddepending on no particular frequency. Therefore, in thehigh-magnification image in which the energy distribution of the spatialfrequency is previously offset on a lower-frequency band, sincecomponents in a high-frequency band of the spatial frequency aresuppressed, the S/N can be readily improved.

On the other hand, in the low-magnification image in which the largeenergy distribution is offset to a high-frequency band, such aprocessing is difficult to be performed.

Note that as a method for improving the S/N by utilizing a differencebetween the spatial-frequency distribution of noise and a spatialfrequency distribution of the target, a method called “waveletshrinkage” is known. Herein, an advantage of performing an S/N improvingprocessing to the high-magnification image has been described in thesequence of automatic pickup of defect images. However, similarlythereto, since the S/N improving processing including a largecalculation amount is performed only to the high-magnification image,the S/N improving processing can be readily performed also in the lengthmeasurement SEM. The reason that the S/N improving processing includingthe large calculation amount can be readily performed to thehigh-magnification image is as follows. That is, for example in thesequence of FIG. 2, in the case of taking the low-magnification image,immediately after the low-magnification image is taken in the step S212,specifying the defect position in the step S213 is required andtherefore no spare time is left. However, in the case of taking thehigh-magnification image, after the high-magnification image is taken inthe step S218, the stage has to be moved and therefore a spare time ispresent.

By using the above-described methods, even if the high-magnificationimage is taken in a state in which the beam current is small, the takenimage can be converted into a good image. Therefore, the image detectioncan be performed in a state in which the frame addition is small, sothat the image detecting time can be shortened.

Therefore, in the SEMs and the image detecting methods according to thepresent invention, the electron beam can be switched within a shorttime, and both the short-time low-magnification image detection and thehigh-resolution high-magnification image detection can be performed.

As described above, the invention made by the present inventors has beenspecifically explained based on the embodiments. However, needless tosay, the present invention is not limited to the above-mentionedembodiments and may be variously altered and modified within a scope ofnot departing from the gist thereof.

For example, although the review SEM and the length measurement SEM havebeen described in the above-described embodiments, the present inventionis not limited thereto and can be applied to other SEMs.

The present invention as described above can be applied to SEMsemiconductor wafer inspection apparatuses, review SEMs, and lengthmeasurement SEMs, etc.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, and the scope of the invention being indicated bythe appended claims rather than by the foregoing description and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

1. A scanning electron microscope comprising: an electron beam sourcefor emitting an electron beam; an electron beam converging means forconverging said electron beam on an observation target surface; a beamcurrent control means for controlling a beam current of said electronbeam; an electron beam scanning means for making said electron beambeing scanned on said observation target surface; a scanning positioncorrection means for controlling a scanning position of said electronbeam scanning means; an observation target moving means for moving saidobservation target so that said observation target surface is within ascanning range of said electron beam by said electron beam scanningmeans; an electron detector for detecting a secondary electron or areflected electron generated by scanning said electron beam on saidobservation target surface; an image formation means for forming animage based on a detection value of said electron detector; a signalgain adjusting means for adjusting a corresponding relation between thesecondary electron or reflected electron detected by said electrondetector and said image formation means; a control amount memory meansfor saving in advance an adjustment amount of said signal gain adjustingmeans so that the detection value of said electron detector falls withina predetermined range in setting a plurality of beam currents or forsaving a calculation algorithm of the adjustment amount; and an imagedetecting condition control means for changing an image detectingcondition in accordance with a change in the beam current by using saidadjustment amount or said calculation algorithm obtaining for theadjustment amount, which is saved in said control amount memory means.2. The scanning electron microscope according to claim 1, wherein thebeam current of said electron beam in said beam current control means iscontrolled by changing transmittance of said electron beam in anirradiation path of said electron beam.
 3. The scanning electronmicroscope according to claim 2, wherein said beam current control meanshas an electron beam transmission device in which setting of thetransmittance of said electron beam is different depending on aposition, and an electron beam transmission device position changingunit for changing the setting position of said electron beamtransmission device to change the transmittance of said electron beam.4. The scanning electron microscope according to claim 2, wherein saidbeam current control means has an electron beam transmission device inwhich setting of the transmittance of said electron beam is differentdepending on a position, and an electron beam deflecting unit forcontrolling the transmittance of said electron beam by offsetting saidelectron beam to change a position where said electron beam passesthrough said electron beam transmission device.
 5. The scanning electronmicroscope according to claim 3, wherein said electron beam transmissiondevice has a heating unit for preventing contamination involved intransmission and shutoff of said electron beam.
 6. The scanning electronmicroscope according to claim 1, wherein, further in said control amountmemory means, a control amount of said scanning position correctionmeans or a calculation algorithm of the control amount for realizingobservation of a same part of said observation target surface in settingthe plurality of beam currents is served.
 7. An image detecting methodof a scanning electron microscope, the method comprising: a step ofinputting coordinates of defects detected by an inspection apparatus forinspecting an observation target surface, selecting one of thecoordinates of said defects, and moving said selected defect into animage detecting visual field of the scanning electron microscope; afirst image detecting step of making an electron beam converged into aspot-like shape scanning said observation target surface, and convertinga secondary electron or reflected electron generated by the scan into anelectrical signal to form an image; a defect position specifying step ofspecifying a defect position on said observation target surface based onthe image formed in said first image detecting step; a second imagedetecting step of making an electron beam converged into a spot-likeshape scanning the defect position specified in said defect positionspecifying step at a magnification larger than that used in said firstimage detecting step, and converting a secondary electron or reflectedelectron generated by the scan into an electrical signal to form animage; a first beam current changing step of observing said observationtarget surface in the second image detecting step by using a beamcurrent smaller than that used in said first image detecting step; afirst image detecting condition setting step of changing an imagedetecting condition to a condition previously set in taking an image atthe beam current set in said first beam current changing step; a secondbeam current changing step of setting a beam current employed in saidfirst image detecting step; and a second image detecting conditionsetting step of setting the image detecting condition employed in saidfirst image detecting step again, wherein said first beam currentchanging step and said first image detecting condition setting step areperformed before said second image detecting step, and said second beamcurrent changing step and said second image detecting condition settingstep are performed after said second image detecting step.
 8. The imagedetecting method of a scanning electron microscope according to claim 7,wherein the image detecting condition of said first image detectingcondition setting step includes at least one of: a corresponding controlamount with respect to said electron beam for suppressing, regardless ofa change of the beam current by said first beam current changing step,brightness of the image generated in said second image detecting step soas to be brighter than brightness of the image generated in said firstimage detecting step; a corresponding relation control amount betweenthe secondary electron or reflected electron generated in said scan formaking said electron beam being converged in a spot-like shape on saidobservation target surface and brightness of an image to be formed; ascanning position correction amount for making said electron beamscanned at a portion specified on said observation target surface; andan electron beam convergence correction amount for making said electronbeam being converged into a spot-like shape on said observation targetsurface.
 9. The image detection method of a scanning electron microscopeaccording to claim 7, further comprising an S/N increasing step ofperforming, to the image formed in said second image detecting step, ahigh S/N increasing image processing for improving a low S/N withrespect to the image formed in said first image detecting step.
 10. Theimage detection method of a scanning electron microscope according toclaim 7, wherein, in said first beam current changing step and saidsecond beam current changing step, a stop or an electron beamtransmission device for controlling transmittance of said electron beamis inserted in an irradiation path of said electron beam so as to changesaid beam current.
 11. The image detection method of a scanning electronmicroscope according to claim 7, wherein a sampling number of the imagetaken in said second image detecting step is larger than a samplingnumber of the image taken in said first image detecting step.